The present invention relates to an installation technique for optical parts and electric parts. More particularly, this invention relates to an installation structure and an installation method for optical modules that are used in the optical communication field.
Conventionally, the positioning of an optical waveguide and an optical element has prevented cost reduction in the process of manufacturing an optical module. Particularly, accurate positioning in the order of micrometers (xcexcm) has been required to achieve an optical coupling between an optical waveguide and a laser diode (LD) as an optical element. As an installation method for easily carrying out this positioning, there is a self-alignment installation method for the positioning by utilizing the surface tension of solder.
According to this installation method, a plurality of metal pads that become the basis of positioning are formed on a substrate, and solder bumps are prepared on these metal pads. On the optical part that is to be mounted on the substrate, metal pads that become the basis for the positioning are also provided corresponding to these metal pads on the substrate. Next, the optical part is provisionally mounted on the substrate based on rough positioning such that the metal pads are brought into contact with the solder bumps on the substrate. Last, the solder bumps are fused by heating the substrate, and the centers of the metal pads on the optical part side are attracted to the centers of the metal pads on the substrate side, based on the surface tension of the solder. As a result, the substrate and the optical part are positioned together with high precision.
According to this installation method, in order to carry out the positioning of an optical part and a substrate to be mounted with this optical part in higher precision, it is effective to reduce the diameter of each metal pad (Shingaku Giho, OQE93-145, 1993, pp. 61-66, (literature 1)). In the mean time, from the viewpoint of achieving compactness and integration, it has also been demanded to reduce the diameters of metal pads on the electronic part to which a similar self-alignment installation technique is applied. However, when the metal pads are made smaller, the permissible positioning error for the provisional mounting becomes smaller. This has had a problem of increasing the installation cost.
Further, as a modification of the above self-alignment installation method, Japanese Examined Patent Publication No. 6-26227 and Japanese Unexamined Patent Publication No. 9-181208 disclose an electric part installation method that permits a large permissible positioning error for the provisional mounting, by increasing the sizes of pads at the four corners.
FIG. 1 shows one example of this conventional part installation structure.
Referring to FIG. 1, a substrate 10 on which a square optical chip part 20 is to be mounted has four large pads 11-1 for rough positioning corresponding to four corners of this optical chip part 20. On the periphery and center of this substrate 10, a large number of small pads 11-2 are provided for fine positioning. Solder bumps are provided on these pads 11-1 and 11-2 respectively. In the meantime, the optical chip part 20 is also provided with large pads 21-1 and small pads 21-2 corresponding to the pads on the substrate. In this example, the optical chip part 20 is placed on the substrate 10, with an angular deviation of xcex8 from a normal mounting position of the optical chip part 20 on the substrate.
According to this installation method, when the large pads 11-1 and the corresponding large pads 21-1 are in contact with each other via the solder bumps to provisionally mount the optical chip part 20, the optical chip part 20 moves to a position where the small pads 21-1 of the optical chip part 20 are brought into contact with the corresponding solder bumps of the substrate based on the surface tension of the bumps of the large pads of the optical chip part 20. All the pads are accurately positioned finally based on the surface tension of all the bumps.
However, when the optical chip part 20 is disposed on the substrate 10, with an angular deviation of xcex8 from the normal mounting position on the substrate as shown in this example, the above positioning is not achieved. In order to provide larger tolerance for this angular deviation, it is necessary to further increase the sizes of the large pads 11-1 and 21-1. In this case, the disposition area of the small pads 11-2 and 21-2 becomes smaller, and the number of these pads that can be disposed becomes smaller. Therefore, this method has had a problem in that the positional precision in the final positioning becomes lower.
In the light of the above problems, it is, therefore, an object of the present invention to provide a part installation structure that can permit larger tolerance of angular deviation in the provisional mounting as compared with that of the conventional technique, and that can realize the positioning with high precision.
According to a part installation structure of the present invention, it is possible to guarantee large tolerance of angular deviation for the part mounting, even when the conventional sizes of large pads are made smaller and/or the number of these pads is decreased. Therefore, it becomes possible to dispose small pads, in an increased number, in the increased area for disposing pads other than the large pads. As a result, it becomes easy to realize fine positioning in the order of xcexcm that is necessary for the installation of an optical part. Further, according to the part installation structure of the present invention, there is provided an effective radiation structure for an optical part via a large metal pad that is disposed in the vicinity of the center of the optical part that has the large generation of heat of a laser diode or the like.
According to one aspect of the present invention, there is provided a part installation structure comprising: a substrate on which a plurality of first metal pads are formed; a part to be mounted having second metal pads corresponding to the positions of the first metal pads; and solder bumps for connecting between the first and second metal pads to match the installation positions of these metal pads based on the surface tension of the solder bumps when the solder bumps are fused, wherein the substrate and/or the mounted part has at least two metal pads having larger areas than those of other metal pads, in the vicinity of the center of the substrate and/or the mounted part.
According to another aspect of the present invention, there is provided a part installation structure comprising: a substrate on which a plurality of first metal pads are formed; a laser diode optical part having second metal pads corresponding to the positions of the first metal pads; and solder bumps for connecting between the first and second metal pads to match the installation positions of these metal pads based on the surface tension of the solder bumps when the solder bumps are fused, wherein the laser diode optical part has at least two metal pads having larger areas than those of other metal pads, in the vicinity of the center of the laser diode optical part, with the metal pads having large areas disposed immediately below an active layer of the laser diode optical part.
According to still another aspect of the present invention, there is provided a part installation method comprising the steps of: patterning a plurality of metal pads on a substrate and a part to be mounted respectively at corresponding positions, and preparing at least two metal pads having larger areas than those of other metal pads, in the vicinity of the center of the substrate and/or the mounted part; preparing solder bumps on the metal pads of the substrate or the mounted part; carrying out rough positioning such that the large metal pads are brought into contact with the facing solder bumps; provisionally fixing the large metal pads and the solder bumps together, by heating the substrate to a temperature at which the solder bumps are not fused; carrying out fine positioning of all the facing metal pads including the other metal pads based on the surface tension of the solder bumps, by heating the substrate to a temperature at which the solder bumps are fused; and cooling the solder bumps by stopping the heating, thereby to complete the fixed installation.